Recently, the growing use of digital control and increase in speed of inverter frequencies allows a welding device working using inverter control to have welding output with various waveforms.
Such a welding device performing arc welding using inverter control has an inverter circuit of a full-bridge structure or a half-bridge structure. Switching elements that form a bridge are power semiconductor devices, such as an IGBT (Insulated Gate Bipolar Transistor) and a MOSFET (Metal-Oxide Semiconductor Field Effect Transistor). Such a power semiconductor device is generally driven by inverter frequencies ranging from several kilohertz to 400 kilohertz, and controls the primary current conduction width of a transformer connected to an inverter circuit, so that welding output is obtained.
As an inverter control method, a PWM (Pulse Width Modulation) method is well known (see Patent Literature 1, for example).
A conventional welding device working on inverter control will be described with reference to FIG. 7. Welding device 101 of FIG. 7 has primary rectifier 118, smoothing capacitor 119, switching section 102, transformer 120, secondary rectifier 121, current detector 103, setting section 108, controller 105, driver 107, and triangular-wave generator 127. Switching section 102 has first switching element TR1, second switching element TR2, third switching element TR3, and fourth switching element TR4. Controller 105 has output comparator 109 and calculator 110.
Welding device 101 is connected to external device 126 such as a power switchboard to get power supply. Besides, welding device 101 has connection to base metal 122 and torch 123 to feed them with welding output. Torch 123 has electrode 124. Welding device 101 supplies electrode 124 and base metal 122 with welding output to generate arc 125 between them, by which welding is performed on base metal 122. The “welding output” mentioned in the description collectively means welding current and welding voltage fed from the welding device.
The workings of welding device 101 having the aforementioned structure will be described below, taking a consumable-electrode type arc welding device as an example.
In FIG. 7, AC voltage, which is fed from external device 126 to welding device 101, is rectified by primary rectifier 118 and then converted into DC voltage by smoothing capacitor 119. The DC voltage fed from smoothing capacitor 119 is further converted, through inverter driving by switching section 102, into high-frequency AC voltage suitable for welding. The high-frequency AC voltage having undergone conversion in switching section 102 is fed into transformer 120 to have transformation. Switching section 102 is formed of first switching element TR1 through fourth switching element TR4 of IGBT. First switching element TR1 through fourth switching element TR4 are switched on/off on PWM method in response to an instruction from driver 107. Switching section 102 thus performs inverter operation. The high-frequency AC voltage fed from transformer 120 is rectified by secondary rectifier 121 formed of, for example, a diode.
One of the welding output from welding device 101 is supplied, via a contact tip (not shown) disposed inside torch 123, to electrode 124 that is a consumable electrode wire. Wire feeding motor (not shown) feeds the consumable electrode wire to torch 123. The other of the welding output is fed to base metal 122. With the structure above, applying voltage between the tip of electrode 124 and base metal 122 generates arc 125, by which welding is performed on base metal 122.
Current detector 103 formed of, for example, a CT (Current Transformer) detects welding current and outputs it. Setting section 108 outputs set current suitable for setting output. Controller 105 receives the set current fed from setting section 108 and the welding current fed from current detector 103. Output comparator 109 of controller 105 calculates difference between the set current and the welding current, and outputs the current difference. According to the current difference, calculator 110 of controller 105 calculates an output-on period of switching section 102 and outputs it. In this way, through feedback control on welding current, controller 105 outputs an appropriate output-on period to driver 107.
Switching section 102 are operated under pulse control by driver 107 according to a cycle determined by a reference triangular wave generated by triangular-wave generator 127 and an output-on period fed from calculator 110 of controller 105. The pulses used for the pulse control on switching section 102 are separated into two lines every other pulse. Specifically, driver 107 outputs the first drive signal and the second drive signal (as the signals separated into two lines) to switching section 102. Triangular-wave generator 127 generates a reference triangular wave that determines an inverter frequency. Driver 107 outputs pulse-width-controlled drive signals in a way that the signals (separated into the two lines) are fed alternately with a cycle that corresponds to the reciprocal of an inverter frequency. The first drive signal synchronizes first switching element TR1 with fourth switching element TR4 to control their on/off, while the second drive signal synchronizes second switching element TR2 with third switching element TR3 to control their on/off.
According to the conventional inverter control described above, a low inverter frequency increases the ripple factor of welding voltage, and causes arc interruption, thereby degrading welding performance. Increasing the inductance (L value) of a DC reactor (i.e. DCL, not shown) disposed on the output side of welding device 101 may be effective in decreasing the ripple factor of welding voltage. However, an increased L value of DCL can fail to produce various welding output waveforms with steep changes.
Preferably, the inverter frequency should be kept high, whereas the ripple factor of welding voltage and the L value of DCL should be kept low. However, increasing an inverter frequency also increases a switching loss of a switching element, thereby heating up switching section 102. This has caused a necessity of countermeasures against heat.